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Module 3 : Sequence Components and Fault Analysis

Lecture 10 : Sequence Components

10.1	Introduction
	Electrical systems occasionally experience short circuits. These short circuits are hazardous to the safety of both equipment and people. Though the protective devices will isolate the faults safely, the parts of the system should withstand the resulting mechanical and thermal stresses. Fault impedance and fault current estimates also form input for the setting and coordination of protective devices like overcurrent relay, distance relay etc. Hence it is very important to estimate the magnitude of the fault currents. The equipment rating are decided based on this value. Fault currents can be estimated either by hand calculation or by fault analysis program.
	Sources of Fault Current
	The fault current in a system can be contributed by any of the following.
	Synchronous Motors and Condensers
	Induction Machines
	Synchronous Generators
	Electrical Utility System
	Distributed Generation
10.2	Sequence Components
	Faults in a 3 phase system can be single line to ground, double line to ground, line to line or three phase. Power system operation during any of these faults can be analyzed using sequence components. The method of sequence component was discovered by Charles L. Fortescue in 1918. He came up with the following intuition that any unbalanced system has 6 degrees of freedom; whereas, a balanced system has only 2 degrees of freedom. Hence an unbalanced system having 6 degrees of freedom can be synthesized by 3 sets of balanced system each having 2 degrees of freedom. Note: This idea can be easily extended to N-phase system where . For a three phase system with phase sequence a-b-c, the three sets of balanced phasors are called positive, negative and zero sequences.